When the unrehearsed, democratically minded dance troupe that Will & Grace viewers know as Debra Messing's face gets to quarreling with itself about where everything should land next, every other actress on TV seems to be wearing a mask by comparison. Not only do you see her thinking, you see her registering in a flash that, as usual, reflectiveness isn't going to help her worth a damn. What makes Grace the perfect sitcom heroine for these excessively briefed, motivationally addled times is that she keeps deciding to be impetuous--usually as a last resort and one that never works any better than her other instant stratagems for coping. It just turns her disasters into a form of self-expression.

You can picture Messing sitting next to you on the subway as you silently wonder if this is the woman of your dreams or someone who ought to be committed. Or maybe, unnervingly, both. By TV standards, she's as unconventional a beauty as Julie Christie or Jeanne Moreau--if not more so, since neither of those sixties screen goddesses had much of a penchant for slapstick. When it comes to pratfalls, Messing is Lucille Ball without the masochism. In her Clairol ads, where she's supposed to appear serene and blandly radiant, she seems unrecognizable even to herself; like Christie's and Moreau's, her kind of beauty isn't made for repose. Her looks are a mobile map of a temperament so unsettled that each of its attributes needs a qualifier: She's neurotic but never self-hating, vulnerable but never a victim, foolish but never dumb, and so on. She can act hilariously sorry for herself--and yet if she ever turned it on for real, her potential for melancholy and remorse might run deeper than Will & Grace could handle.

Even short of that, Messing's volatility is so clearly without reliable frontiers that her responses, which are as tangled as her hair, keep adding an unexpectedly lifelike note of emotional anarchy to a series that's designed as an efficient, smoothly running joke machine. (The only other TV actress with this quality is That '70s Show's Laura Prepon, who plays Donna; the look in her eyes keeps saying that she knows things the show doesn't, and Prepon's show isn't even all that dumb.) One reason that even the best sitcoms are seldom very sexy is that sexiness is an unruly quality pretty much by definition, and joke machines depend on behavior that never hops the grooves. That's why it's a tribute to Messing that the writers had to invent a howling zany like Megan Mullally's Karen just to make Grace seem like the normal one. No matter how smug Will & Grace can be about its own sophistication in making homosexuality safe for prime-time laughs, Messing's presence keeps reminding you that sitcoms have barely tackled female heterosexuality yet. In her erotic scheming and frustration, she's a miniature Scarlett O'Hara.

Without her, Will & Grace would be as disposable as it is clever. But without its expert engineering, she wouldn't be as effective as she is; she needs contrast. Unlike Jenna Elfman, say, who always knows exactly what she's up to, Messing isn't entirely in control of how she comes across on the screen, which is why she's more interesting to watch. She seems to be as impulsive as an actress as Grace is as a character, which shows just how hard it can be to tell where performance leaves off and personality kicks in--especially on a weekly series, where sheer familiarity makes viewers so intimate with every last mannerism and tic. In roles that don't turn her on, she's a cipher. She was no more than competent in her previous sitcom, Ned and Stacey, and not even that in CBS's Jesus miniseries this spring--even if playing Mary Magdalene, a part that could leave Vanessa Redgrave looking sheepish, isn't exactly a fair test of anybody's acting skills. For all that, though, she's unique and something splendidly unprecedented on TV--a sitcom actress that FranÃffÃ,Â§ois Truffaut would have lost his heart to faster than you can say Jules and Jim.